The Collaborative International Dictionary of English v.0.48:

Nunc dimittis \Nunc" di*mit"tis\ [L. nunc now + dimittis thou
   lettest depart.] (Eccl.)
   The song of Simeon (--Luke ii. 29-32), used in the ritual
   of many churches. It begins with these words in the Vulgate.
   [Webster 1913 Suppl.]